This proposal is to study basic neural mechanisms of the brain and spinal cord that are used in the control of motor output. The experiments will be carried out with macaque monkeys that are trained to execute a general trasking task; the monkeys are required to compensate for changes in force and to track changes in target position. The dorsal roots associated with a given limb will be transected in order to test the hypothesis that stiffness, rather than muscle length or force individually is regulated by the actions of autogenetic reflexes. Deficits in the abilities of these animals to make reaction-time movements in response to either proprioceptive or visual stimuli will also be assessed. A major portion of the proposed experiments is directed toward a microelectrode study of the role of magnocellular and parvocellular neurons of the red nucleus in the control of muscle length and tension. Specifically, relationships between discharge rate, limb position and force will be evaluated; stimulus-response properties will be studied particularly with proprioceptive stimuli; and an attempt will be made to evaluate the role of this nucleus in controlling both motoservo actions and reaction-time movements.